I think I can help with that. There's a little metal tab (in the red circle in the attached picture) that gets pressed in when the film door is closed. When you open the door, that tab is released, resetting the frame counter. It's possible that some debris got around that tab, jamming it. Try clearing any debris out and seeing if that solves the problem. You should also be able to press the tab in with a screwdriver. Good luck!
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